Permeation and its impact on Packaging


The migration of a gas or vapor through a packaging material

Impact of permeation

Shelf life is the length of time that foods, beverages, pharmaceutical drugs, chemicals, and many other perishable items are given before they are considered unsuitable for sale, use, or consumption. Permeation greatly influences the shelf life of these products as the loss or gain of oxygen, water vapor, carbon dioxide and odors and aromas can rob the product of flavor, color, texture, taste, and nutrition. Oxygen, for example, causes adverse reactions in many foods such as potato chips. By measuring the rate at which O2 permeates through the package material, one can begin to determine the shelf-life or amount of time the unopened package will still provide ‘good’ chips.

Permeation measurements are crucial to targeting a package’s sweet spot. This sweet spot is defined as the intersection between effective marketing, product protection and cost. When all three areas combine, a functional, cost-effective package that meets its shelf life requirements is developed. However, the product protection segment is often overlooked and can have a significant impact on the other segments, especially the cost.
Product protection through packaging encompasses both physical and chemical safeguard. While protection of the product from physical damage is always a factor, chemical protection must also be considered. Microbiological contamination, oxidation, rancidity, loss of moisture, component degradation, flavor loss, off-flavor, and loss of carbonation are just some examples of the chemical damage that can be caused without appropriate packaging. Permeation measurements are key in defining the appropriate package to help minimize or control this damage.

Understanding the permeation rates of a packaging material in the beginning of the package development process helps to avoid both over- and under-packaging. Both scenarios can ultimately be quite costly but in most cases can be avoided with a proper testing protocol.Under-packaging (inadequate barriers, improper gauges, wrong material choices, etc.) allows the transmission of some compound(s) at a rate that causes product degradation faster than the desired shelf-life. Repercussions from under-packaging can range from product complaints and returns all the way potentially to voided warranties, law suits and legal action.

Over-packaging probably will not result in legal action but can be a significant waste of money and material resources. Often times, a lack of product knowledge will lead a manufacturer to use the best package available within a given budget in order to prevent under-packaging.
FIGURE 1b. Package design versus environmental Impact
FIGURE 1b. Package design versus environmental Impact.
If the proper testing program had been implemented pre-launch, significant dollars could have been saved in wasted costly over-packaging. Furthermore, once the product is a success, it’s often impossible to change anything connected with the package design for fear of losing market share. At either end of the spectrum, package design can have a potentially large impact on the environment. As Figure 1b. illustrates, over-packaging results in excessive material usage, thus negatively impacting the environment. Under-packaging creates product returns resulting in increased waste, again creating a negative impact on the environment.

Barrier materials

The knowledge and level of sophistication in the barrier plastic packaging business has skyrocketed as absolute barrier type packages such as cans and bottles has shifted to permeable packaging systems. 1983 marked the introduction of a co-extruded, multi-layer plastic ketchup bottle to the US supermarket shelves. Since then, companies have been working at a feverish pace on packaging strategy. The competition is steep and product quality and package design could be the keys to market success.

Decision-makers have an increasingly new variety of resins, materials, package configurations and technologies to evaluate. Barrier levels of materials today range from ultra-barriers used in the electronics industry to breathable, perforated high transmitters for packaging produce. Standard materials such as polyethylene, PET and EVOH are sharing the spotlight with newcomers such as PGA and liquid crystal polymers.

Using permeation data in the package development process

The following steps outline the recommended method for determining the product protection segment of the package’s sweet spot.

Identify Product Requirements

Understand how to develop a package specification. Not only does the maximum amount of allowable product degradation need to be specified, the contributors to degradation must also be identified. While certainly not an easy task, product requirements are a key piece of information in the development of a successful package. While oxygen and water vapor ingress and/or egress are common, other gasses such as carbon dioxide, odors and aromas must also be considered.

Identify candidate materials

With so many new barrier packaging materials and resins available, it is often difficult to compare permeation data as derived from material suppliers’ data sheets. A comprehensive materials testing program can be implemented which will enable the selection of the best barrier material for a particular application. Beginning with transmission rate testing of candidate sheet materials to both confirm suppliers’ data as well as narrow the candidate field, final package configurations can be created. The packages can then be tested to determine the overall barrier level.

Determine Optimal System

Product requirements and candidate materials are then matched to either determine the shelf life obtainable in a specific material, or to determine which material will supply a specified shelf life. Storage or shelf life studies are usually then conducted to confirm the package performance.

What is permeation?

Graham’s Colloidal Diffusion illustrates the three-step process of permeation through a material. The first step is the sorption of the permeant into the material. Next, the permeant diffuses through the material followed by the final step where it is desorbed from the other side. This entire process is driven by a concentration difference or gradient. Molecules will permeate from the high concentration side of the material to the low concentration side. This will continue until both sides are at equal concentration levels.
Permeation measurement is generally a straight-forward concept. However, terminology surrounding permeation and its measurement is often used incorrectly. Permeation rate and transmission rate are used interchangeably but are, in fact different properties.

The rate at which the permeant goes through a material of a specific area is evaluated to give the transmission rate. The units of transmission rate have the dimensions of volume of permeant per unit area per time. Some examples of transmission rate units are:

Wath is Permeation?

Transmission rate is a measurement specific to the actual material tested, permeant and conditions such as temperature and relative humidity (RH).

When more general results are required, the permeation rate is calculated from the measured transmission rate data. Permeation rates take into account the material thickness and the driving force or concentration gradient of the permeant. Permeation rate provides more of a material property as it can be used for any given material thickness or permeant driving force. To obtain the permeation rate, the measured transmission rate results are multiplied by the thickness of the material tested and divided by the partial pressure gradient of the permeant. Of course, many instruments automatically calculate the permeation rate and provide it as one of the outputs. Some examples of permeation rate units are:

Wath is Permeation?

Units for thickness, area and pressure may be switched between SI and metric.

Measurement techniques

Most common techniques for transmission rate measurement of sheets include challenging one side of the material with the permeant (test gas) while the other side is swept with a carrier gas. The material is placed in a diffusion cell, separating it into two chambers. The inner chamber is flushed with nitrogen carrier gas and the outer chamber contains the permeant. Molecules of permeant diffuse through the film to the inside chamber and are carried to the sensor by the carrier gas. The computer monitors the increase in water vapor or gas concentration in the carrier gas and it reports that value as the transmission rate (FIGURES 2a and 2b.) This process is perfectly illustrated in the transmission rate units shown above; amount of permeant per unit area per time. Although the typical test sample size is only 50 cm2 it is normalized to either 1 m2 or 100 in2 for purposes of the units. A variety of permeant-specific sensors are used to analyze the concentration in the carrier gas stream. Different sensors can be used depending on the sensitivity required due to barrier level as well as the accuracy desired.
FIGURE 2a. Schematic of an oxygen transmission rate test
FIGURE 2a. Schematic of an oxygen transmission rate test.
Transmission rate data from flat samples is extremely useful in initial material evaluations, research and development applications and ranking potential materials as candidates for a given package. However, final package configurations can and should also be analyzed. Testing the transmission rate of packages provides insight into the stresses incurred in the packaging process. Full package testing is typically recommended when developing shelf life predictions.
FIGURE 2b. Schematic of a water vapor transmission rate test
FIGURE 2b. Schematic of a water vapor transmission rate test.
FIGURE 3a. Diagram of an oxygen package transmission rate test
FIGURE 3a. Diagram of an oxygen package transmission rate test.

Package transmission rate tests (FIGURE 3a) are conducted under the same principal as sheet measurements. The test gas is supplied to either the interior or exterior of the package while the carrier gas sweeps the opposite side. Typically in the test gas inside / carrier gas outside scenario, a capture volume is required (FIGURE 3b). Since thickness may vary throughout a package and the actual area may be difficult to calculate, the standard transmission rate units for a package are:


FIGURE 4. Graph of a transmission rate measurement versus time
FIGURE 4. Graph of a transmission rate measurement versus time.

Transmission rate testing does not provide an instantaneous result. For both sheets and packages, it is a dynamic test. Once the test has been started, there is a time of transient behavior prior to reaching equilibrium (FIGURE 4). This time to equilibrium varies between materials, permeants and temperatures. Excellent barriers can take weeks to reach equilibrium while high transmitters might take only a couple hours.

Important factors in permeation measurement

While concentration gradient is the driving force behind permeation, other factors play an important role. Temperature has a large effect on permeation rates as the permeation rate increase 5-7% per degree C (FIGURE 5). Correct temperature control and measurement is crucial to obtaining accurate results.
FIGURE 5. Effect of temperature on permeation rate
FIGURE 5. Effect of temperature on permeation rate
Package testing can be especially susceptible to temperature variation as many packages are tested in ambient air and the temperature in many labs can fluctuate several degrees in a 24-hour period.

Relative humidity (RH) can also impact permeation results. Obviously when testing WVTR, the proper RH generation, control and monitoring are crucial as the water vapor serves as the test gas. However, the presence of moisture can also greatly impact the permeation rates of oxygen and other gasses in certain materials. Figure 6 shows the effect of moisture on the OTR of some key materials used in the packaging industry. Proper RH generation and control during an OTR test provides an accurate indication of how the material will truly perform under those conditions.

FIGURE 6. Effect of RH on oxygen transmission rate of EVOH
FIGURE 6. Effect of RH on oxygen transmission rate of EVOH.

Other factors to consider when testing transmission rates include sheet thickness variation, equilibrium time, barometric pressure and proper test gas generation.


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